Abstract
A Pliocene-Pleistocene cross-peninsular marine seaway has been proposed to explain genetic divergence between northern and southern populations of land, plant, and animal species along the Baja California peninsula. The location of the hypothetical seaway is the low-lying San Ignacio Trough that trends NE-SW across the central Baja California peninsula. Although the seaway hypothesis has gained favor in genetic studies, it has not been tested with geologic data. In this study, we tested the seaway hypothesis through a detailed stratigraphic, sedimentological, geochronologic, and ichnological study of deposits that are well exposed at Mesa San Regis approximately 12 km east of San Ignacio. The informally named San Regis beds are at least 27 m thick, and not more than approximately 40 m thick. They contain interbedded tephra dated at 4.29+ or -0.04 Ma, and are capped by a Mg-andesite flow dated at 3.70+ or -0.01 Ma ( (super 40) Ar/ (super 39) Ar). Four measured sections reveal stacked, fining-up intervals of pebbly sand to silt and mud with abundant calcrete and few preserved sedimentary structures. Calcic paleosols commonly occur at the tops of fining-up intervals and display varying stages of development. Ichnofossils (i.e., burrows) include Naktodemasis, Coprinisphaera, Celliforma, Parawanichnus, and Megaichnus, formed by various soil bugs, beetles, bees, ants, and large mammals, respectively, and common rhizocretions, rhizotubules, and rhizohaloes formed from roots of groundcover and shrubs. No marine fossils have been observed. The presence of stacked paleosol sequences, cross-bedding, and terrestrial trace fossils indicates that the San Regis beds accumulated in broad channels, point bars, and floodplains of a low-gradient intermittent river system. Ichnofossils, paleosols, and sedimentology of the San Regis beds are all diagnostic of deposition in a terrestrial environment, in a semi-arid climate with ample plants where evaporation was greater than precipitation to promote accumulation of pedogenic carbonate. All sedimentary deposits younger than the San Regis beds are thin and accumulated in alluvial, fluvial, and playa lake depositional settings. Collectively, these findings refute the hypothesis for a cross-peninsular seaway and suggest that alternate hypotheses, such as glacial refugia and regional rainfall variations, could explain the genetic divergence pattern.